I'll give Maxim some credit, because the idea of the minimap stemmed from him.
Anyways, the idea is if you press shift, you'll get this UI (in multiplayer):
http://i.imgur.com/ulninHu.png
On the left, it would show the player list, it'd have more info than normal PB2. It'd show the player's skin, and have boot (X), view (V), and profile (P). It would show some simple stats at the very right of the list, like ping and K/D. Names would appear above the XVP stuff, the XVP buttons would be much smaller. At the very bottom of the list would show the time you've been playing, the map ID and map name, with a few other stats.
On the right is the actual minimap. The minimap, as implied, shows a miniature map of the world. In this map, you could see players, the direction they face, some types of other entities such as barrels and executable buttons. The option for players to be shown on the map could be turned off and on via triggers and engine marks.
In singleplayer, pressing shift would bring up only the map, but it'd take up more of the screen. Undiscovered areas could be blurred out by triggers or engine marks.
Minimaps in both gamemodes would be able to be zoomed in and out of. In singleplayer, pressing shift once would lock the minimap open and pause the game. Pressing it again would resume the game and remove the minimap. In multiplayer, the game would always run and shift would not lock, so you'd have to hold shift to look at the minimap and player list.
So that's my idea on minimaps and the in-game UI in PB2.5. What are everyone else's thoughts on this?